Pipe snubbing units typically have vertically spaced gripping heads mounted on a frame in a manner such that the lower head remains stationary while the upper head is raised and lowered through the action of hydraulic jacks or the like. The gripping heads each carry slip members that can be alternately engaged with and released from the pipe being run into or withdrawn from the well, and one of the heads may be constructed and arranged to cause rotation of the pipe.
As shown and described in U.S. Pat. No. 4,269,277, a gripping head may include a plurality of circumferentially spaced and radially shiftable carrier blocks on which slip members are mounted. The slip members normally are centered with respect to the carrier blocks by spring-loaded plungers that bear against transverse pins on the blocks. The carrier blocks and slip members are shifted inwardly until teeth on the inner surfaces of each slip member are brought into gripping contact with outer surfaces of the pipe. Each carrier block has upper and lower oppositely inclined wedge surfaces that are cooperable with companion wedge surfaces on a respective slip member in a manner such that either upward or downward relative movement of the pipe increases the gripping pressure. However should one of the slip members move vertically relative to the others during operation of the gripping unit, so that the slip members are not precisely in the same horizontal plane, the loading on the slip members will not be uniform which can result in a condition where the pipe can be damaged by crushing or the like.
It is the general object of the present invention to provide a new and improved pipe gripping unit of the type described.
Another object of the present invention is to provide a new and improved pipe gripping unit having means for ensuring uniform engagement of the teeth of each slip member with the outer surface of the pipe.